It is well known in the prior art to incorporate low coherence optical interferometers into various optical coherence tomography ("OCT") apparatus to study scattering media. A basic form of such an OCT apparatus found in the prior art comprises an interferometer that includes a 50/50 beamsplitter, or a 3 dB coupler if the interferometer is embodied using optical fibers. In a typical prior art optical fiber embodiment of the OCT apparatus, a low coherence radiation source and a photodetector are coupled to two input ends of the 3 dB coupler. The beams of radiation transmitted from two output ends of the 3 dB coupler are transmitted to a sample medium to be tested and a reference medium, respectively. The beams from the output ends are: (a) reflected from the sample medium and the reference medium, respectively; (b) combined by the 3 dB coupler; and (c) transmitted to the photodetector. As is well known in the prior art, when the optical pathlength mismatch between radiation reflected from the sample medium and radiation reflected from the reference medium is less than the coherence length of the low coherence radiation source, measurable interference occurs between these the two beams. Then, if the optical pathlength of the radiation reflected from the reference medium is known, when the photodetector senses the interference signals, the optical pathlength of the radiation reflected from the sample medium can be measured to the accuracy of the coherence length of the radiation source.
It is also known in the prior art to utilize OCT methods and apparatus to investigate the eye. In doing so, several types of apparatus have been used to provide a reference medium to facilitate measurement of the optical pathlength of radiation reflected from the reference medium. For example, an OCT apparatus disclosed in an article entitled "Optical Coherence Tomography" by David Huang et al., Science, Vol. 254, pp. 1178-1181, Nov. 22, 1991 utilized a mirror to reflect a reference beam back to a photodetector. In the disclosed OCT apparatus, depth information for radiation reflected by the sample medium is acquired on a step-by-step basis by moving the mirror with a stepper motor. The disclosed OCT apparatus has been modified in the art, for example, see U.S. Pat. No. 5,321,501. U.S. Pat. No. 5,321,501 discloses: (a) the use of a retroreflector instead of the mirror to improve the optical alignment stability and (b) the use of a galvanometer instead of the stepper motor to increase the scan speed. The increased scan speed is important because it makes it feasible to obtain tomographical images of living tissue. In this regard, in vivo human eye retinal tomography has been demonstrated in an article entitled "In vivo retinal imaging by optical coherence tomography" by Eric Swanson, et al., Optics Letters, Vol. 18, No. 21, pp. 1864-1866, Nov. 1, 1993. A disadvantage of such OCT apparatus is that the depth measurement is limited to about 3 mm to 5 mm for in vivo human eye measurement.
An article entitled "Coherent optical tomography of microscopic inhomogeneities in biological tissues" by V. M. Gelikonov et al., JETP Lett., Vol. 61, No. 2, pp. 158-162, Jan. 25, 1995 disclosed the use of a piezoelectric radial actuator with a fixed mirror in the reference arm of an interferometer to fabricate an OCT apparatus. In this OCT apparatus, the optical pathlength of the reference arm is modulated by applying a signal to the piezoelectric actuator, thereby stretching the optical fiber longitudinally. In this arrangement, although the scan speed can be improved, the scan depth is still limited. Further, stretching an optical fiber to increase the scan depth causes other problems such as birefringence and hysteresis.
In light of the above, there is a need for a method and apparatus for simply and economically providing efficient scanning in an OCT apparatus.
In addition to the above, there is a need to utilize the efficient scanning apparatus to perform eye length measurements. At present, eye length measurements are made by measuring the delay of an ultrasound echo back from the retina. Due to the attenuation of ultrasound energy in the eye, only low frequency ultrasound energy can be used. As a result, the accuracy is typically only about 200 .mu.m. This accuracy represents a measurement error for refraction of approximately 1/2 diopter and is considered significant in clinical applications such as cataract surgery. Further, this measurement technique suffers because the method requires contact with the eye (this is not comfortable for a patient).
In light of the above, there is a need in the art for method and apparatus for accurately measuring the length of an eye, preferably in a non-contact mode.
An article entitled "Optical Measurement of the Axial Eye Length by Laser Doppler Interferometry" by C. K. Hitzenberger, Investigative Ophthalmology & Visual Science, Vol. 32, No. 3, Mar. 3, 1991, pp. 616-624 discloses the use of dual beam Michelson interferometry to measure the eye length using a low coherence light source. The disadvantage of the disclosed configuration is that a bifocal lens is required to focus the beams on the cornea and the retina separately. If this were not done, the signal strength is too weak for imaging the retina. Embodiments of the present invention provide an alternative interferometer configuration to measure eye length without physically increasing the reference beam scan length. In addition, such embodiments are capable of scanning the retinal image with a good signal to noise ratio.